Methyl isocyanate, ethyl isocyanate and other low boiling isocyanates are difficult to store and transport due to their high vapor pressure, toxicity and flammability. Even such compounds as phenyl isocyanate and the isomeric tolyl isocyanates which boil at temperatures from 167.degree. C. to about 200.degree. C. present storage and transportation problems.
Many compounds are known to react with isocyanates through double bond addition to the N.dbd.C bond. Reaction with alcohols from N-methylcarbamoyl esters which are relatively stable compounds. The reaction of naphthyl isocyanate with phenols is a classic method of characterizing phenols.
It has been conventional practice to obtain low boiling isocyanates from the corresponding carbamoyl chloride by heating in an appropriate solvent inert to isocyanate while separating the hydrochloric acid from the gaseous isocyanate. A recent U.S. Pat. No. 4,082,787 makes a review of various processes.
U.S. Pat. No. 4,003,938 describes a method for obtaining low boiling isocyanates by decomposing the corresponding .beta.-naphthyl carbamates at temperatures of from 150.degree. C. to 500.degree. C. This process, however, presents difficulties as the .beta.-naphthyl carbamate is a solid at room temperature and the reaction requires a vacuum.
The low boiling isocyanates are considered to be alkyl isocyanates having from 1 to 10 carbon atoms, preferably from 1 to 4 carbon atoms, in the alkyl and alkenyl isocyanates having from 2 to 6 carbon atoms in the alkenyl. Such compounds have the boiling points given in Table I.
TABLE I ______________________________________ Isocyanate Boiling Point .degree.C. ______________________________________ Methyl 38-45 Ethyl 60 Isopropyl 74 n-Propyl 88 Allyl 88 n-Butyl 115 t.-Butyl 85 CH.sub.3 --C(CH.sub.3).sub.2 --CH(CH.sub.3)-- 58 (13 mm) di-(n-propyl)-methyl 40 (12 mm) ______________________________________
U.S. Pat. No. 4,146,550 describes a process for preparation of aliphatic monoisocyanates by reacting the corresponding carbamic acid chloride with sulfuric acid or sulfonic acid amides, certain activated phenols containing electrophilic groups and certain substituted urethanes and then subjecting the reaction product to pyrolysis at temperatures of from 100.degree. C. to 250.degree. C. to recover the aliphatic isocyanates. This process requires the use of a solvent, such as chlorobenzene, and the aliphatic isocyanate and solvent are recovered as a mixture which must be carefully redistilled to recover the aliphatic isocyanate.